The results from ACCRETE are given in the 26 peer-reviewed articles listed below. Many of the articles, including figures, can be obtained electronically from the journal web sites, and/or from the author web sites. The text below summarizes the major results.

Highlights of ACCRETE geological results

Andronicos, et al (1999) showed that dextral transpression occurred across the Coast Mountains batholith during the time of proposed northwards translation of Baja BC (mid-Cretaceous-Paleocene). Truncations of seismic reflectors, contrasts of physical properties, and offset of Moho (Morozov et al, 1998, 2001, 2003) showed that a nearly vertical structure passes through the crust near the Coast shear zone. The combined results of geologic and seismologic studies therefore suggest that a crustal scale dextral, transcurrent, strike-slip shear zone occurs within the Coast Mountains batholith, and the present Coast shear zone is near the western boundary of this dextral shear zone. Based on its size, age, and sense of displacement, this dextral shear zone could represent a section of the Baja BC fault system. Motion across this shear zone ceased by 58 Ma.

A major and unanticipated result from ACCRETE was to show that crustal scale extension of the Coast Mountains batholith occurred during the Eocene. This extension was concurrent with intrusion of large volumes of Eocene granodiorite to diorite plutons (Hollister and Andronicos, 1997; Chardon, et al, 1999; Andronicos, et al, 2003). It was also concurrent with 15-20 km of exhumation. Thanks to ACCRETE and a rapidly expanding awareness of the large region affected by the Eocene magmatism and extension, it is becoming increasingly apparent that this Eocene episode was an important event for forming the continental crust of western North America.

Hollister et al (2004) showed that the intense episode of magmatic accretion between 50 and 60 Ma in the Coast Mountains (Andronicos et al, 2003) led to reheating of portions of the Baja BC terrane. This reheating reset magnetic remanence directions in the older rocks when ilmenite exsolved from ilmenohematite, leading to lamellar magnetism. Although the reset directions are consistent with what would be expected from tilting or folding (Butler et al, 2001; 2002), Hollister et al (2004) showed that older magnetic remanence directions were reset towards those corresponding to the Eocene magnetic pole. These results strengthen the Baja BC hypothesis.

Summary of ACCRETE geophysical results

The strong S-wave generation in the ACCRETE experiment provided S-wave coverage comparable to that of P- waves (Morozov et al, 2001). The S-wave generation was probably produced by mode conversion at the glacially polished interface between unconsolidated, poorly sorted glacial sedimentary deposits and the underlying bedrock. This is a feature presumably unique to where glaciers once extended from the continental margin to the sea. The unusual S-wave coverage allowed us to obtain a model of Vp/Vs velocity ratios, from which much better constraints on lower crustal lithologies could be interpreted than from a Vp model alone (Morozov et al, 2003).

The geophysical experiments showed that the thickness of continental crust under the Coast Mountains batholith is only 31-32 km (Morozov et al, 1998; Hammer et al, 2000). This was a first-order surprise that is contributing to the multitude of hypotheses and studies regarding extension across western Canada.

The seismic velocity structure indicated a lower crust made of a mixture of mafic garnet granulite and restite. The mafic garnet granulite is the metamorphic equivalent of gabbro at the present P-T conditions at the base of the crust, and the restite is what remains after extraction of granitic melt (Morozov, et al, 2003). We proposed that these lithologies represent what remained after basalt liquid had intruded the lower crust and had partially melted it. Mixing of basalt and melted crust probably resulted in formation of the plutons of the Coast Mountains batholith.

The average crustal velocity under the Coast Mountains batholith resembles that of average continental crust when the loss of 10 - 20 km by unroofing is considered. Unroofing by that amount was proposed by Hollister (1982) and confirmed and refined by Andronicos, et al (2003).

Educational outreach

A program of educational outreach evolved from the extensive permitting process necessary for the marine seismic survey (Hollister, 1995, 1997, 1998). A geological guide for the interested public is in preparation. Help for producing this guide has come from a wide range of local citizens, and it has been extensively tested with groups representative of potential users.

21. Butler, Robert F.; Gehrels, George E.; Baldwin, Suzanne L.; Davidson, Cameron, 2002, Paleomagnetism and geochronology of the Ecstall pluton in the Coast Mountains of British Columbia: Evidence for local deformation rather than large-scale transport. J. Geophysical Research 10.1029/2001JB000270

The ACCRETE program is a collaborative
endeavor to determine how continents grow by magmatic and terrane accretion,
and to investigate the relative roles of these processes. The focus is
on the Coast Mountains orogen of southeast Alaska and British Columbia.
The central segment of the Coast Mountains orogen is an ideal laboratory
because (1) we have acquired geophysical data across it that are exceptional
in quality and quantity, (2) rocks presently exposed at the surface were
formed at mid-crustal levels, and (3) we can project mid-crustal geological
features from the surface down-dip into the seismic section. Combining
geophysical and geological data we will construct a section across the
orogen from the surface to the upper mantle. Besides providing answers
to fundamental questions regarding continental growth at convergent to
transpressive plate margins, we also address major questions regarding
the overall tectonic history of western North America.

ACCRETE was funded from the Continental Dynamics program of NSF,
for 1996-1999. The funds were for continued processing of our high quality
seismic data and for geologic and geochemical studies along the seismic
transect. There are 9 PIs at 8 institutions.

Click on image for larger scale.

Above: Ship tracks for the EWING cruise EW 9412. Unreadable numbers
on lines are common depth points (CDPs). Approximate location of Coast
shear zone is shown for reference. The shaded area schematically shows
locations of wide-angle midpoints for which the Univ. of Wyoming and Univ.
of British Columbia teams observed Moho reflections. Track lines between
133 and 135 degrees West cover the Queen Charlotte transform fault (contact
Rohr or Trehu).

Below: Ship track of EWING within the ACCRETE core corridor,
showing lines 1251, 1252, and 1255. Line 1251 was shot at 20 second intervals
from south to north end of Portland Canal, and at 60 second intervals on
return to the entrance to Portland Inlet. Line 1255 was shot from the entrance
to Portland Inlet to the west at 20 second intervals. Unreadable numbers
along lines are CDPs.